Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process

Tetrasulfone[9]helicene (PTSH) was newly synthesized to improve and evaluate its fluorescence and excited-state dynamics through a single-step oxidation reaction of tetrathia[9]helicene (PTTH). In electrochemical measurements, the reduction potential of PTSH was shifted in a positive direction by approximately 1.0 V when compared to that of PTTH because of its electron-accepting sulfone units. The of the electrochemical measurements agree with the energy levels calculated by density functional theory (DFT) methods and steady-state spectroscopic measurements. Furthermore, a significant enhancement of the absolute fluorescence quantum yield (Phi(FL)) was achieved. The absolute fluorescence quantum yield of PTSH attained 0.27, which is approximately 10 times larger than that of PTTH (Phi(FL) = 0.03). Such an enhancement of Phi(FL) can be successfully explained by the corresponding kinetic comparison. The reason is mainly the increased energy gap Delta E-ST between the lowest singlet (S-1) and triplet (T-1) excited states. Finally, excellent circularly polarized luminescence of PTSH was also observed. The value of the anisotropy factor g(CPL) was estimated to be 8.3 X 10(-4) in PTSH.